Syntheses and applications of nano-sized iron particles

ABSTRACT

The present invention relates to syntheses of nano-sized iron particles formed by mixing a source of ferrous iron with a combination of reducing agent and base, and the particles formed therefrom.

FIELD OF THE INVENTION

The present invention relates to methods to synthesize nano-sized iron particles and applications thereof.

BACKGROUND OF THE INVENTION

Zero valent nano iron particles (i.e., below about 100 nm), because of their unique properties, find use in many technology areas, including but not limited to environmental remediation, bioseparation, security packaging and electronics. One typical way to produce iron particles of this size is to convert ferrous or ferric iron into zero valent iron by using strong reducing agents such as NaBH₄. However, NaBH₄ is generally expensive and yields of zero valent iron are generally low.

U.S. Pat. No. 6,242,663 describes a method for remediation of heavy metals and halogenated hydrocarbon contaminants from aqueous media using iron solutions that are reduced to form zerovalent nano iron particles. The reduction is accomplished by use of large amounts of NaBH₄, but without any additional base.

C. Wang, et al., Environmental Science and Technology, 1997, 31, 2154, describes the synthesis of nanosized iron particles for use in dechlorination of halogenated organic compounds. The synthesis employs NaBH₄ as a reducing agent, but again no additional base is used.

F. Li, et al., Colloids and Surfaces A: Physicochem. Eng. Aspects 223 (2003) 103-112, describes microemulsion and solution approaches to nanoparticle iron production for the degradation of trichloroethylene. The synthesis employs NaBH₄ as a reducing agent, but again no additional base is used.

There is a need in the industry for a lower cost novel method to make nano-sized zerovalent iron particles.

SUMMARY OF THE INVENTION

The present invention relates to a process to produce nano-sized zerovalent iron particles, comprising the steps of:

a) forming a first aqueous solution comprising ferrous or ferric ions;

b) forming a second aqueous solution comprising a reducing agent and a base; and

c) mixing said first and second solutions together, thereby precipitating nano-sized zerovalent iron particles.

The present invention further relates to particles made by the novel process.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a SEM photograph of particles made as described in Comparative Example A.

FIG. 2 is a SEM photograph of particles made as described in Example 3.

FIG. 3 is a SEM photograph of particles made as described in Example 4.

FIGS. 4 a and 4 b are SEM photographs of particles made as described in Example 8.

FIGS. 5 a and 5 b are of TEM photographs of particles made as described in Example 3.

FIG. 6 is a schematic drawing of the continuous process as described in Examples 9 to 11.

DETAILS OF THE INVENTION

The present invention relates to processes for forming nano-sized iron particles. These processes generally have higher yields and lower cost of manufacturing compared to other processes in general use. Additionally, the processes described herein allow the control of particle morphology, size and specific surface area, thereby allowing control over the formed particles' reactivity, magnetic properties and suitability for certain end-uses such as environmental remediation.

Nano-sized zero valent iron particles formed by the process described herein exhibit higher reactivity compared to nano iron particles formed from processes that do not add a base solution.

In the present invention, nano-sized zero valent iron is produced by reducing ferric or ferrous iron, generally in the form of a chloride or sulfate, using borohydride, selected from NaBH₄, KBH₄, and LiBH₄; and a base, generally selected from NH₄OH, NaOH, KOH and mixtures thereof. The invention could alternatively use a salt in place of the base. The salt may be selected from carbonate, bicarbonate, borate and phosphate. By the addition of a base in the process, the yield of nano iron based on the amount of reductant borohydride increases substantially. The process allows the morphology of the nano iron particles to be selected, depending on the reaction conditions. For example, it is possible, by using the novel process, to produce spherical, needle-like or flower-like nano iron particles which alters the surface area of the particles produced.

The process begins by forming an iron-containing solution of an aqueous solution of a ferrous, ferric iron or mixtures thereof. The iron particles are found in the range of 1 g/l to 284 g/l, with a preferred range of 10˜100 g/l.

In a separate container, a solution of a reducting agent selected from NaBH₄, KBH₄, LiBH₄, and a base selected from NH₄OH, NaOH, KOH and mixtures thereof is prepared forming a reducing agent/base solution. The base can be added separately to the reducing agent, or may be supplied as part of a commercially-available source. The concentration of reducing agent is in the range of 0.1 g/l to 150 g/l. The concentration of base is 0.1 g/l to 400 g/l. The molar ratio of the reducing agent to base is in the range of 0.1 to 10. The iron-containing solution and the reducing agent/base solution are then mixed together. The zero-valent iron particles precipitate out of solution to form a slurry. The slurry is then filtered and the zero-valent iron particles are collected in any convenient manner. One particularly convenient manner is as a filter cake.

The ferrous iron solution and the reducing agent solution is kept chilled and at least substantially free from oxygen. This is generally accomplished by using cold, deoxygenated water to produce the solutions. The solutions are generally kept under nitrogen or other inert gas to prevent oxidation of the materials. The solution is kept at least substantially free from oxygen because the presence of oxygen will consume some reductant and thus reduce the yield.

As shown in the examples below, the yield of nano-size iron particles, based on the amount of reductant used, greatly increases when a base, such as NH₄OH is used. For example, it is theorized that NH₄OH, when used as a co-reactant, neutralizes the H⁺ generated by the reaction as shown in Equation 1, so that the reaction is driven toward formation of zero-valent iron: 4Fe²⁺(aq)+BH₄ ⁻(aq)+3H₂O→4Fe(s)+H₃BO₃(aq)+7H⁺(aq)  Eq. 1 Without the addition of NH₄OH or another base, Fe(s) will further react with H⁺ and thus consume much of the zero-valent iron by the reaction shown in Equation 2: Fe(s)+2H⁺→Fe²⁺+H₂↑  Eq. 2 With the presence of enough NH₄OH or other base, the reaction can be rewritten as shown in Equation 3 to achieve the maximum yield: 4Fe²⁺(aq)+BH₄ ⁻(aq)+7NH₄OH→4Fe(s)+H₃BO₃(aq)+7NH₄ ⁺(aq)+4H₂O  (Eq. 3) This increase in yield (greater than 8 times the process without additional base) is shown in the examples below.

The process of the present invention can be a batch or continuous process. FIG. 6 is a flow diagram showing a schematic whereby the iron and reductant/base solutions are added to a reactor, where the zero-valent iron particles are formed. In the continuous process discussed in the examples below, NaOH is the base used with the NaBH₄ reducing agent. However, this was done for convenience only; NaOH. NH₄OH, KOH or mixtures thereof may be used in either batch or continuous mode.

The morphology of the formed iron particles is dependent upon the amount of base used. It was found the amount of base and the ratio of base to reducing agent will affect the morphology. As shown in the examples below, zero-valent iron particles formed without the addition of base produce generally spherical particles. By adding various amounts of base to the reducing solution, spherical, needle-like and flower-like morphologies can be produced. The non-spherical morphologies (e.g., needle-like, flower-like) have higher surface areas, e.g., from 25 m²/g to 99.5 m²/g as measured by the BET method (S. Brunauer, P. H. Emmett and E. Teller, J. Amer. Chem. Soc., 60, 309 (1938)). The different shapes can find use in different end-uses as well. For example, when a higher surface area is needed to more completely remediate a contaminated waste stream, it may be preferable to use needle-like or flower-like morphology particles.

Unless otherwise specified, all chemicals and reagents were used as received from Aldrich Chemical Co., Milwaukee, Wis.

EXAMPLES Example A (Comparative)

This example demonstrates the production of Fe particles in the presence of NaBH₄, but in the absence of a base, such as NH₄OH.

De-ionized water was bubbled with nitrogen for at least 30 min before use. The de-oxygenated water was then transferred into a freezer and kept inside for 30 min. FeCl₂ solution was prepared by dissolving 11.13 g ferrous chloride tetrahydrate with 200 ml cold deoxygenated water in a 1000 ml Erlenmyer flask. In a separate container, reducing solution was prepared by dissolving 3.78 g NaBH₄ solid in 200 ml cold de-oxygenated water. The reducing solution was then slowly pumped into ferrous chloride solution using a peristaltic pump with strong agitation. A black precipitation was observed immediately. After finishing pumping the reducing solution, the slurry was stirred for at least 5 minutes. The slurry was then filtered using a 0.2 micron membrane filter. The black cake was rinsed with 100 ml ethanol. The total weight of wet cake was 4.25 g. The wet cake was then kept in a refrigerator to avoid oxidization. A small amount of sample was analyzed to determine the surface area by BET, metals analysis by ICP, and particle morphology by SEM. ICP metal analysis showed this sample contained 51.8% iron. The surface area was 52.0 m²/g by BET measurement. The SEM picture (FIG. 1) showed spherical morphology with diameter of about 40 to about 120 nm. The yield based on gram iron produced by per gram NaBH₄ was 0.65.

Example 1

De-ionized water was bubbled with nitrogen for at least 30 min before use. The de-oxygenated water was then transferred into a freezer and kept inside for 30 min. FeCl₂ solution was prepared by dissolving 11.13 g ferrous chloride tetrahydrate with 200 ml cold deoxygenated water in a 1000 ml Erlenmyer flask. In a separate container, reducing solution was prepared by dissolving 3.78 g NaBH₄ solid and 7.57 g ammonium hydroxide solution (˜28.9% NH₃ in water) in 200 ml cold de-oxygenated water. The reducing solution was then slowly pumped into the ferrous chloride solution using a peristaltic pump with strong agitation. A black precipitation was observed immediately. After finishing pumping the reducing solution, the slurry was stirred for at least 5 minutes. The slurry was then filtered using a 0.2 micron membrane filter. The black cake was rinsed with 100 ml ethanol. The total weight of the wet cake was 4.56 g. The wet cake was then kept in a refrigerator to avoid oxidization. A small amount of sample was sent to measure surface area by BET, and to analyze for metals by ICP. ICP metal analysis showed this sample contained 65.7% iron. The surface area was 69.8 m²/g by BET measurement. The yield based on gram iron produced by per gram BH₄ was 0.79.

Examples 2 to 8

The procedures of Examples 2 to 8 are similar to that of Example 1, except the amount of NaBH₄ and NH₄OH used were different. The amounts of NaBH₄ and NH₄OH used in Examples 3 to 9 are shown in Table 1. The results of these examples, along with Comparative Example A and Example 1 are summarized in Table 1. SEM photographs of Example 3 and Example 4 are shown in FIG. 2 and FIG. 3 respectively. FIGS. 4 a and 4 b are SEM photos of the sample from Example 8. FIGS. 5 a and 5 b show the TEM photos of needle-like morphology of particles from Example 3.

The yield based on NaBH₄ was increased from 0.65 kg Fe/kg NaBH₄ (Comparative Example A) to 5.95 kg Fe/kg NaBH₄ (Example 8) by using NH₄OH as co-reactant. TABLE 1 Yield NH₄OH Wet BET Weight (kg Ex. NaBH₄ 28.9% Sample Surface ICP Fe of iron Fe/kg No. (g) (g) (g) Area(m²/g) (%) produced NaBH₄) Morphology A 3.78 0.00 4.25 52.0 58.1% 2.47 0.65 Spherical 1 3.78 7.57 4.56 69.8 65.7% 3.00 0.79 2 1.06 3.79 8.85 98.9 26.3% 2.33 2.20 3 1.06 7.57 13.47 122.4 23.1% 3.11 2.94 Needle- like 4 1.06 11.36 6.57 96.7 44.5% 2.92 2.76 Flower 5 0.53 0.00 1.45 25.7 20.8% 0.30 0.57 6 0.53 3.79 7.20 95.9 40.1% 2.89 5.45 7 0.53 7.57 11.43 99.5 22.1% 2.53 4.77 8 0.53 11.36 16.50 76.7 19.1% 3.15 5.95 Flower- like

Examples 9-11

The following examples demonstrate the use of NaOH in conjunction with NaBH₄ in the continuous production of Fe particles.

As shown in FIG. 6, iron chloride tetrahydrate (FeCl₂.4H₂O) (Aldrich) was dissolved in de-oxygenated water (produced by sparging deionized water with nitrogen overnight) and charged to a feed tank. NaBH₄ (12 wt % stabilized with 40 wt % NaOH in water from Aldrich Chemical Co., Milwaukee, Wis.) was used as received and charged to a feed tank. Nitrogen was supplied to each feed tank in an amount sufficient to push the materials to the reactor. Flow rates of the materials were controlled by rotameters.

Black metallic iron was formed immediately upon mixing of the FeCl₂ and NaBH₄ solutions. This product was collected by two different methods. Method 1 used a conventional filter to recover the Fe from the solution. Method 2 used a decanter-settler. In both cases, the vessel with the recovered Fe in it was transferred to a nitrogen box where the Fe was discharged into bottles for storage. Flow rates and other process parameters are given in Table 2. Yield was determined by mass balances using the weights of Fe and NaBH₄ fed and the amount of Fe product collected as measured by ICP. The results of examples 9-11 were summarized in table 3. TABLE 2 Example# 9 10 11 Iron Source FeCl₂*4H₂O (g) 741 741 743 Water (g) 12981 12981 3843 Total (g) 13722 13722 4586 Fe concentration (%) 1.52% 1.52% 4.55% Feed rate (cc/min) 37 37 37 Amount fed (g) 13722 13722 4370 Boron source 12% NaBH₄/40% NaOH/H₂O 183 183 366 (g) Water (g) 1044 1044 454 Total (g) 1227 1227 820 NaBH₄ concentration (%) 1.79% 1.79% 5.36% Feed rate (cc/min) 5 5 6 Amount fed (g) 964 1227 337 Product Recovery Collection method filter decanter decanter Wet cake total (g) 198 212 171

TABLE 3 Yield Wet BET Weight (kg NaBH4 NaOH Sample Surface ICP Fe of iron Fe/kg Example # (g) (g) (g) Area(m{circumflex over ( )}2/g) (%) produced NaBH4)  9 17.25 57.50 198.00 75.2 49.3% 97.6 5.66 10 21.96 73.20 212.00 78.0 55.7% 118.1 5.38 11 18.06 60.20 171.00 66.0 56.0% 95.8 5.30 

1. A process to produce nano-sized zerovalent iron particles, comprising the steps of: a) forming a first aqueous solution comprising ferrous or ferric ions; b) forming a second aqueous solution comprising a reducing agent and a base; and c) mixing said first and second solutions together, thereby precipitating nano-sized zerovalent iron particles.
 2. The process of claim 1, wherein said first aqueous solution comprising ferrous ions is formed by dissolving ferrous chloride in deoxygenated water, and wherein the concentration of said ferrous ions is 1 g/l to 284 g/l.
 3. The process of claim 1, wherein said first aqueous solution comprising ferrous ions is formed by dissolving ferrous chloride in deoxygenated water, and wherein the concentration of said ferrous ions is 10 g/l to 100 g/l.
 4. The process of claim 1, wherein said reducing agent of said second solution is borohydride selected from NaBH4, KBH4, and LiBH4.
 5. The process of claim 1, wherein the concentration of said reducing agent is 0.1 g/l to 150 g/l.
 6. The process of claim 1, wherein said base is selected from NaOH, NH₄OH, KOH or mixtures thereof.
 7. The process of claim 5, wherein the concentration of said base is 0.1 to 400 g/l.
 8. The process of claim 1, wherein the molar ratio of said reducing agent to said base is in the range of 0.1 to
 10. 9. The process of claim 1, wherein said mixing and particle formation are done in a batch mode.
 10. The process of claim 1, wherein said mixing and particle formation are done in a continuous process.
 11. Nano-sized zero valent iron particles formed by the process of claim
 1. 12. Nano-sized zerovalent iron particles formed by the process of claim 1 wherein the iron particles exhibit a spherical morphology.
 13. Nano-sized zerovalent iron particles formed by the process of claim 1 wherein the iron particles exhibit a needle-like morphology.
 14. Nano-sized zerovalent iron particles formed by the process of claim 1 wherein the iron particles exhibit a flower-like morphology. 